Aneuploidy is defined as an alteration in chromosome number that is not a multiple of the haploid complement. Such karyotypic changes are frequently associated with human disease. Aneuploidy is the leading cause of miscarriages and mental retardation in humans. It is also prominently associated with cancer: more than 90% of all solid human tumors are aneuploid. Understanding the relationship between aneuploidy and tumorigenesis is thus vital if we want to make progress in our battle against this disease. The studies proposed here will contribute towards this goal. We have previously examined the effects of aneuploidy on cell physiology by analyzing the effects of additional chromosomes on yeast cells. We observe that the presence of extra chromosomes impairs cell proliferation, causes phenotypes that are indicative of proteotoxic stress and results in genome instability. All these phenotypes are due to the proteins produced from the additional chromosomes. We will now determine the molecular bases of these phenotypes in established aneuploid yeast strains and determine the kinetics with which these phenotypes arise using inducible aneuploidy systems. In Specific Aim 1 we will characterize protein aggregates in aneuploid yeast strains. We will furthermore test the hypothesis that chromosome imbalances in aneuploids lead to protein stoichiometry imbalances, which cause proteotoxic stress and an increased burden on the cell's protein quality control systems. In Specific Aim 2 we will further investigate how aneuploidy brings about genome instability. Specifically, we will determine how aneuploidy causes an increase in recombinatorial repair. In Specific Aim 3 we will develop an inducible chromosome mis-segregation system that allows us to create more complex aneuploidies and to study the immediate consequences of aneuploidy on the cell. Together, we believe that these studies will shed light on how aneuploidy affects cell physiology and how the condition contributes to tumorigenesis.